> You're saying that people [knew] exactly what slavery was like, they just> didn't realize it was "immoral"? I find this hard to fathom. I> instead see two major changes from the past:> 1) People were getting richer, and richer folks can better afford to> indulge their "moral" feelings. > 2) U.S. slave were unusual in history: the slave population grew locally.> In contrast, most slaves were spoils of war/domination imported from> elsewhere, and who didn't reproduce very well locally. Its lots> easier to be cruel to foreigners than to locals.

Both of those seem plausible, but I still think the most fundamental
underlying change is moral belief itself. People believe all kinds of
crazy things; why is it so hard to accept that they believed any of a
number of justifications for the common practice of slavery: racial
inferiority, economic necessity, right of conquest. It is part of our
mental machinery to de-sensitize ourselves to everyday things.

Let's take an example from modern times: circumcision. Objectively,
it is absolutely no different from female genital mutilation as
practiced in Muslim nations. It has no medical basis (though you
will hear many doctors give post-hoc justifications for it that have
more evasive maneuvers than a submarine), it is provably painful to
the infant (and many still ignore that fact and claim otherwise).
In short, it is useless, barbaric, cruel, ... and commonplace. I'm
not saying that moels and OBs are heartless machines; most are likely
very compassionate people. But they were brought up with certain
mystical beliefs, and never challenged them with proper reasoning.
But the moel knows what is involved--he sees the blood, he hears the
cries, he has just used belief to immunize his mind from the facts.
He simply does not "know" that it is immoral, because he has failed
to ask the question and rationally pursue the answer.